The invention relates to a material based on perovskites as thermally insulating layers or heat barriers for the protection of thermally loaded or stressed substrates, especially for use in a gas turbine.
To increase the efficiency of stationary and cantilevered gas turbines, at the present time increasingly higher gas temperatures are being used in these machines. For this purpose components of the turbine are provided with thermally insulating layers (TIL) which as a rule are comprised of yttrium stabilized zirconium oxides (YSZ). A bond-promoting layer (BL) of an MCrAlY alloy (M=Co, Ni) or an aluminide layer can be provided between the substrate and the thermally insulating layer to serve primarily as oxidation protection for the substrate. With these systems to date, surface temperatures of the turbine components up to 1200xc2x0 C. can be utilized.
A further increase to above 1300xc2x0 C. is desirable but cannot be achieved with the existing materials, especially with YSZ. The zirconium oxide which is deposited by plasma spraying or electron beam vapor deposition undergoes at temperatures above 1200xc2x0 C. a phase transformation which within the operating life gives rise to deterioration of the coating. At equal thermal conductivities of the heat insulation layer and equal layer thicknesses, higher surface temperatures give rise to higher temperatures in the bonding layer and the substrate. These temperature increases also contribute to an accelerated deterioration of the bond of the material.
As a consequence, new materials are being sought on a world wide basis which can redeem the partially stabilized zirconium oxide as a material for a thermal insulation layer.
The object of the invention is to provide a material for a thermal insulation layer which fulfills the requirements of a low thermal conductivity, a high thermal expansion coefficient and substantially a phase stability to temperatures above 1300xc2x0 C. It is also an object of the invention to provide thermally stressed components with such a thermal insulation layer.
In the framework of the invention it has been found that the oxides of rare earth elements (Sc, Y) which are available in a perovskite structure comprise materials which have especially advantageous characteristics for a thermal insulation or thermal barrier layer.
The material according to the invention is thus characterized by a perovskite structure. This has the general formula ABO3. The A and B positions can include typically a multiplicity of elements. The layer can have at least one element from the group of the lanthanides for the A position or B position.
The lanthanide group together with elements scandium and yttrium also make up the group named as rare earths. The lanthanides are elements with atomic numbers 57-71 of the Periodic System of the Elements.
For the formation of a perovskite structure, cations of different sizes are required for the A and B position. Especially, a large cation is required for the A position and an average size cation for the B position. The oxides of the rare earths and their mixtures (rare earth mixtures) crystallize usually depending upon the ionic diameter and temperature in three different structures, the hexagonal A form, the monoclinic B form and the cubic C form.
Within the framework of the invention it has been found that a rare earth mixture with clearly different ionic radii and with a stoichiometric ratio of about 1:1 will crystallize out in perovskite structure with the general formula ABO3.
A perovskite is thus formed advantageously when in the material the A position is provided with the larger cations of La, Ce or Nd, and the B position includes the cations of Yb, Lu, Er or Tm.
Especially advantageous perovskite structures as LaYO3, LaLuO3, LaErO3, LaTmO3, CeYO3, CeLuO3, CeErO3, CeTmO3, PrYO3, PrLuO3, PrErO3, PrTmO3, NdYO3, NdLuO3, NdErO3, and NdTmO3.
A further advantageous conformation of the material is a mixed perovskite in which the A and/or B positions are provided with at least two different lanthanides. Especially for A=Axe2x80x2=Axe2x80x3=(La, Ce, Pr, Nd) at the A-position and/or B=Bxe2x80x2=Bxe2x80x3=(Er, Tm, Yb, Lu) at the B-position, an especially suitable material will result.
The advantageous perovskite structure of the material of the invention is characterized especially by a high melting temperature. The melting temperature for the material depending upon its composition can be above 1800xc2x0 C. and especially above 2000xc2x0 C. Up to the range in which the material reaches its melt temperature, such materials advantageously do not undergo phase transformation and thus can be employed for corresponding purposes, especially as heat insulating layers.
In a further advantageous conformation of the material, the material has a thermal expansion coefficient in excess of 8.5xc3x9710xe2x88x926 Kxe2x88x921. As a further advantageous feature it also has a thermal conductivity of less than 2.2 W/mk.
A material with these characteristics is especially well suited as a thermal insulation layer on a metal substrate since the matched thermal expansion coefficient reduces mechanical stresses between the two materials upon temperature increase and the reduced thermal conductivity usually prevents an overheating of the substrate.
The component of the invention can have a layer on the surface comprised of a material as has been described.
Such a layer is a highly effective thermal insulating layer for thermally loaded components which also must not undergo phase transformation until well over 1200xc2x0 C. Because of the reduced thermal conductivity of this layer high temperatures are prevented as a rule from reaching the component surface. This permits an efficient operation of the machine and/or a longer useful life of the component.
Advantageously the material of the component and the layer have similar thermal expansion coefficients. This prevents stresses of thermal origin from breaking away the layer from the component surfaces.
Advantageously, between the layer according to the invention and the component, at least one further layer is provided which for example acts as a bond-promoting layer which improves the adhesion between the individual layers and functions as an oxidation protection for the substrate.
A suitable material for such an adhesion promoting layer is an alloy with the general formula MCrAlY. The M in this formula signifies either nickel or cobalt, Cr is chromium, Al stands for aluminum and Y signifies Yttrium. A bond-promoting layer of this material is especially thermally resistant and advantageously matches the thermal expansion coefficients of the layers bordering it.
An intermediate layer of aluminide is also advantageous.
The material according to the invention (lanthanide-perovskite) can advantageously also be used as the uppermost layer in a multilayer system which is applied to a substrate. This multilayer system can be comprised of a bond-promoting layer and at least two further layers. In the simplest case a two-layer system of a first YSZ layer directly on the bond-promoting layer and a further oxide layer, like for example La2Zr2O7, as a second layer can be used.
Also an advantageously continuous transition can be fabricated between these layers in the form of concentration gradients. A suitable embodiment of the component has a layer provided on the surface which has a concentration of lanthanides starting from that at the interface between the component and the layer which increases toward the surface of the layer. Thus this layer has a concentration gradient with respect to the lanthanide.
The thermally insulating layer is advantageously provided on the surfaces of components of a gas turbine. As a result such gas turbines can operate with higher gas temperatures especially gas temperatures above 1200xc2x0 C. Higher gas temperatures mean advantageously an improvement in the efficiency of a gas turbine.